Effects of swirl intensity on interfacial and wall friction factors of annular flows in a vertical pipe

Interfacial and wall friction factors, f(i) and f(w), of air-water swirling annular flows in a vertical pipe of diameter D = 30 mm were measured in three sections of L* (=L/D) = 7.3, 13 and 31, where L is the distance from a swirler. The gas and liquid volumetric fluxes, J(G) and J(L), were 12.5 <= J(G) <= 20.0 m/s and 0.071 <= J(L) <= 0.213 m/s, respectively. The interfacial swirl number, s(i), defined by V-i theta/V-iz, where V-i theta and V-iz are the azimuthal and axial components of interfacial velocity, was introduced as an indicator of swirl intensity. For z* (=z/D) <= 7, where z is the axial coordinate in the test section, the s(i) increased with z* at low J(G) and J(L), and slightly decreased with z* at high J(G) and J(L). The s(i) decreased for z* > 7 and reached zero at z* asymptotic to 30. The f(w) estimated using the three-fluid model was larger than that estimated using the two-fluid model since the droplet flow rate was not negligible even in swirling annular flows. The three-fluid model was therefore more appropriate for evaluation of f(i) and f(w) than the two-fluid model. The ratio f(w)& lowast; increased with (s)overbar i for (s)overbar i > 0.25, where fw & lowast; is the ratio of f(w) to that of non-swirling flows and (s)overbar i is the averaged value of s(i) in the section. The ratio fi & lowast; increased with (s)overbar i for (s)overbar i > 0.18, where f(i)& lowast; is the ratio of f(i) to that of non-swirling flows. Correlations in terms of (s)overbar i have prospect to evaluate the f(i) and f(w) in swirling annular flows.